Natural Gas and Oil Properties (Narrative) (Details)	3 Months Ended
	Mar. 31, 2018 item $ / bbl $ / MMBTU	Mar. 31, 2017 item $ / bbl $ / MMBTU
Natural Gas and Oil Properties [Line Items]
Natural gas, oil and NGL reserves discount	10.00%
Period of time needed to calculate ceiling value of reserves	12 months
Number of hedge positions designated for hedge accounting | item	0	0
Natural Gas [Member] | Henry Hub [Member]
Natural Gas and Oil Properties [Line Items]
Full cost ceiling test, price | $ / MMBTU	3.00	2.73
Oil [Member] | West Texas Intermediate [Member]
Natural Gas and Oil Properties [Line Items]
Full cost ceiling test, price	49.94	44.10
NGL [Member] | West Texas Intermediate [Member]
Natural Gas and Oil Properties [Line Items]
Full cost ceiling test, price	14.90	10.17